1. Field of the Invention
The present invention relates to a process for producing a honeycomb structure, comprising mainly a cordierite crystalline phase, used as a catalyst support in an exhaust gas purifying device.
2. Description of the Related Art
A honeycomb structure, comprising mainly a cordierite crystalline phase, used as a catalyst support in an exhaust gas purifying device, is known (for example, Japanese Unexamined Patent Publication (Kokai) No. 6-165939).
In such a honeycomb structure, if the porosity of the structure is low, the compressive strength is high but the amount of catalyst supported is small. On the other hand, if the porosity of the substrate is high, the amount of catalyst supported is large but the compressive strength is low.
It is therefore difficult to make a honeycomb structure having a high compressive strength as well as able to support a large amount of catalyst.
The object of the present invention is to provide a process for producing a honeycomb structure having a high compressive strength as well as able to support a large amount of catalyst.
The above and other objects of the invention are attained by providing a process for producing a honeycomb structure, comprising the steps of: providing a substrate having cell walls in a honeycomb form and comprising SiO2, Al2O3 and MgO as the main chemical components, applying to said substrate a mixture of a component for lowering the cordierite crystal phase crystallization temperature of said substrate with a liquid medium, and firing said substrate with said mixture applied to form a honeycomb structure comprising a crystalline cordierite phase, said crystalline cordierite phase comprising 42-56% by mass of SiO2, 30-42% by mass of Al2O3 and 12-18% by mass of MgO as the main chemical.
In accordance with the present invention, by applying said mixture of a component for lowering the crystallization temperature of cordierite crystal phase volume of pores having a pore size of 1 xcexcm or less, which do not contribute to the catalyst support, in the honeycomb structure decreases, while the ratio in volume of pores having a pore size of 1 xcexcm or more, which do contribute to the catalyst support, increases. As a result, by decreasing in the ratio of small pores and therefore the total porosity, the compressive strength of the honeycomb structure is increased, while the catalyst supporting capability does not decrease in comparison with the prior art since the ratio of large pores does not decrease.
The reasons for the above are not clear, but it is considered that since the crystallization temperature of cordierite near the surface of the cell walls of the substrate is lowered, the cordierite crystalline phase is formed during the firing step from the surface toward inside of the cell walls of the substrate and, in this gradual crystallization step, the small pores preferentially disappear in comparison with the large pores.
In the present invention, it is preferred that the firing temperature after the application of said mixture is close to a temperature for firing the substrate without the mixture applied. By the firing of the substrate with the applied mixture at the temperature at which the substrate without the mixture applied is to be fired, the small pores more effectively decrease. Here, the firing temperaturexe2x80x9d means the crystallization temperature and xe2x80x9cthe temperature close to the firing temperaturexe2x80x9d can be a temperature at which the substrate can be substantially fired or crystallized.
It is also preferred that the mixture is applied to 80% or more, more preferably 90% or more, of the total surface area of the cell walls of the substrate. By this application at such a large area of the surface of the cell walls of the substrate, the small pores can decrease over approximately the entire surface of the cell walls, by which it is possible that the strength of the fired structure increases and the catalyst supporting capability thereof is excellent.
It is also preferred that after said step of applying of said mixture, an excess portion of said mixture is removed from the surface of said substrate. By this removing step, the mixture can be appropriately uniformly applied or coated to the surface of the cell walls of the honeycomb structure, which is desired.
It is further preferred that the substrate to which the mixture is applied is a substrate which has not been fired yet and the medium of the applied mixture is a non-aqueous organic medium. In this process, since the substrate to which the mixture is applied and which is to be fired is a non-fired substrate, a desired honeycomb structure can be obtained simultaneously when said substrate is fired, i.e., by a single firing step.
The component for lowering the crystallization temperature of cordierite may be one or more of sources for the components constituting the cordierite, such as talc, alumina, kaolin, iron, titanium and mixtures thereof. Alternatively, components which become impurities in the cordierite, such as iron and titanium, can be used as said component for lowing the crystallization temperature of cordierite.
It is also preferred that a substrate having cell walls in a honeycomb form and comprising SiO2, Al2O3 and MgO as the main chemical components is fired to form a fired substrate; a mixture of a component for lowering the cordierite crystal phase crystallization temperature of said fired substrate with a medium is applied to said fired substrate; and said fired substrate with said mixture is again fired to form a honeycomb structure comprising a crystalline cordierite phase, said crystalline cordierite phase comprising 42-56% by mass of SiO2, 30-42% by mass of Al2O3 and 12-18% by mass of MgO as the main chemical components. In this process, a mixture of a component for lowering the crystallization temperature of cordierite crystal phase with a medium is introduced in small pores of cell walls of a once-fired substrate, and the substrate is then fired, by which the small pores decrease. In this process, since the substrate, to which a mixture of a component for lowering the crystallization temperature of cordierite crystal phase with a liquid medium is to be applied, is a once-fired substrate, the substrate is stable and therefore the application of the mixture is easy.